1. Field of the Invention
The present invention relates to actuators, and sensors, and specifically to bending actuators and sensors. More specifically, the present invention relates to the generation of a non-uniform strain-inducing field within a single slab bending actuator and the generation of asymmetric fields in a bending sensor, having a predetermined geometry and non-uniform thickness.
2. Description of Related Art
For a large class of materials, the material properties such as strain and stress, and the like, can be changed and controlled by the application of an external energy, typically in the form of a field. Materials that exhibit these effects are considered “active materials.” As an example, electric, magnetic, and electromagnetic fields induce changes in physical properties of certain materials. More specifically, the application of an electric field to a piezoelectric material changes the overall shape of the material in known, repeatable ways. Electrostrictive, magnetostrictive, and electro-optic materials, are additional examples of active materials. The properties for a mechanically active member material are generally represented by the following strain equation:ε=ΣIki*FI
where,                ε is strain;        kI represents proportionality constants, and,        FI is the applied field strength, raised to the ith power.Essentially, any material that allows for field-induced deformations may be utilized as an active member for an actuator or bender.        
Piezoelectric materials are one example. The following equations represent one form of these relationships:
 S=d·E+s·TD=g·S+ε·Ewhere
S and T are the strain and stress matrices;
E is the electric field;
D is the displacement field vector;
d and g are the piezoelectric strain and field constant matrices, respectively;
s is the mechanical compliance constant; and
ε is the dielectric constant.
Many materials, naturally existing in crystalline form, are piezoelectric. Quartz is one prevalent example. Other materials, such as Barium Titanate, and Lead Zirconate Titanate (PZT) are ceramics, fabricated with randomly oriented piezoelectric domains. The randomness of the domains in an initially fabricated state causes the cancellation of the intrinsic piezoelectric effects for samples large relative to the domain size. However, upon the application of a sufficiently large electric field, called the coercive field, the domains align and the bulk material becomes “piezoelectric”. Furthermore, this field establishes a preferred axis within the material. The act of aligning the domains is called “poling” the material.
Active materials have been utilized to make actuators, acoustic devices, passive filters and resonators, inertial and pressure sensors, and voltage generators and transformers.
Actuators, devices that have the ability to do mechanical work on command, can be constructed from active materials. Typically, actuators comprise mechanical members with defined stiffness, where the architecture of the actuator member is such that upon the application of an external field the active material of the actuator member is deformed. There are a number of distinct classes of actuators: stacked, flex-tensional and “morph” actuators. The various classes exhibit different functional characteristics in terms of actuation principle, actuation distance, often referred to as travel, mechanical compliance, frequency response, and power consumption. The instant invention is directed to actuators that bend as a result of internally induced moments. Hence, devices of the instant invention are improvements on morph style actuators.
The class of active member actuators, called morphs, converts induced strains into moments and causes the active member to bend. Morphs are sometimes referred to as “benders” since the member bends during actuation. Piezoelectric morphs work like bimetallic springs driven by electric fields rather than temperature. In this way, morphs exploit the electric field to stress/strain relationship to form actuators. Morphs utilize coplanar geometries where the attached electrodes are on substantially plane, parallel sides. This coplanarity promotes field uniformity within the device, which in turn, is essential to the device function. Furthermore, morphs are completely unrestrained over at least some large portion of their extent to allow bending. As actuators, morphs are important because of their very large travel during actuation.
Unimorph designs are constructed by bonding a plane, parallel, active slab, such as a piezo-ceramic, to a plane parallel inactive slab. Driving or actuating the active slab generates moments within this multi-slab structure. These generated moments cause the entire unimorph structure to bend. Unique to a unimorph design, only one part of the device, the active slab, is directly subject to, and under the influence of, a voltage potential. Importantly, if an inactive slab is not bonded to an active slab, no moments are generated within the active slab and the active slab does not bend, though it will elongate.
A prototypic unimorph configuration is shown in FIG. 1. One slab of metallized piezoelectric material 2 is poled. As shown in FIG. 1A, it is bonded to an inactive slab 3 along their large planar surfaces with an inner electrode 4 between them. A voltage difference is applied across electrodes 4, 5 to generate an electric field inside the active slab. The result 7 is that a moment is generated causing the bonded pair to bend as depicted in FIG. 1B. Morphs utilize thin slabs, actuate in the device's thickness direction and develop significant internal stresses when actuated. Note that these internal stresses develop even if the material is unloaded during actuation.
Bimorph and multi-slab morph designs are also constructed from plane, parallel, multiple slabs bonded together. In bimorph and multi-slab morph designs; fields are generated in each layer or slab. Bonding two pieces of active substrate together, such as two pieces of piezoelectric material, makes a bimorph, so that differential changes in length of the two pieces can produce relatively large deflection movements perpendicular to the direction of elongation. FIG. 2 depicts a standard bimorph (two-slab) actuator. FIG. 2A depicts a bimorph rectilinear-shaped member 10, first shown without an external field applied. As with all bimorphs, two slabs of active material are bonded together. In this configuration, a voltage difference is applied across three independent electrodes. Voltage is applied between an electrode attached to the top surface 12 and an electrode attached between the two slabs 14. A voltage difference is also applied between electrode 14 and an electrode attached to the bottom surface 16. FIG. 2B denotes a cross-sectional view of the bimorph member 10 of FIG. 2A under an applied electric field. The electric field 19 induced within the member 10 is perpendicular to the surface and parallel to the poling direction 21. Regions of high strain 18 and regions of reverse strain 20 are shown. Moments developed within active member 10 cause the member to bend in order to minimize the total strain energy. FIG. 2C depicts member 10 in its altered shape. High strain region 18 is evident on the top surface, while low strain region 20 is depicted on the bottom surface.
Other bimorph designs exist where two piezoelectric slabs of opposite polarization are bonded together and the applied field maintains the same direction though both slabs, going from an electrode on the top surface of the top slab to the bottom surface of the bottom slab. Multi-layer morphs consist of a plurality of individual slabs of active material, three or more, each with a potentially different applied field and poling direction.
The active slabs used in morph actuators are substantially rectilinear and have rectangular cross-sections. Surfaces of these actuators are plane and parallel such that the slabs have a uniform thickness. As such, electrodes on opposing surfaces induce electric fields that are substantially uniform within active slabs. Field uniformity within each active slab increases the effectiveness of the actuation and hence leads to larger actuation travel for a given applied field. These types of actuators find application in a variety of devices. Typically, they are long, thin, narrow devices, or thin circular diaphragms and membranes. Geometries of these types of devices are structured to maximize the field uniformity within the device.
In U.S. Pat. No. 5,471,721 issued to Haertling on Dec. 5, 1995, entitled, “METHOD FOR MAKING MONOLITHIC PRESSTRESSED CERAMIC DEVICES” a monomorph bender is taught produced by reducing one side of a high lead containing piezoelectric or electrostrictive material by a high temperature chemical process. This high temperature chemical process converts the initially homogeneous member into a non-homogeneous slab having different piezoelectric activity levels through its thickness. This non-uniform piezoelectric activity leads to moments when stimulated with a uniform field. Here, as with the other morphs, uniform fields within the slab are desired as they improve the efficiency of the actuator.
Bonding piezoelectric materials to a metal slab is difficult and, in fact, many morph device failures are attributable to the metal/piezoelectric material interface. Often, this bonding layer occurs near the center of the device thickness and precisely at the location and along the direction of greatest sheer stress within the device.
The large stresses inherently present in the active slabs of morphs have other deleterious effects on actuator function. For example, large stresses contribute to device aging. Aging is the long-term process through which the organizing effects of poling are reversed and the material returns to its randomly organized, inactive state. In addition, the large internal stresses contribute to material failure.
The piezoelectric phenomenon is bidirectional. Therefore, just as electrical energy will cause a mechanical reaction in the form of stress or strain depending on the mechanical boundary conditions, applied mechanical loading in the form of applied stresses or enforced displacements will generate corresponding electrical reactions in the form of charge or voltage depending on the specific electrical boundary conditions. Piezoelectric sensors exploit this mechanical loading to electrical reaction relationship.
Morph devices may also be used as sensors. The principle sensing applications are point loads sensing, distributed loads such as pressures sensing and acceleration sensing through detection of the loading caused by its own mass or the loading caused by a proof mass. The devices themselves are constructed in the same manner as actuators: two regions of different piezoelectric activity are bonded together. In use, the electrodes are connected to measurement circuits instead of driving circuits. As a mechanical load sensor, the morph is configured to experience a bending moment from the signal of interest. The moment establishes a non-zero average stress between opposing electrodes. In turn, this non-zero average stress generates either a voltage difference between the electrodes or excess charge on the electrodes, depending on the measurement circuit used.
The average stress developed between opposite sides of a single layer, plane, parallel, homogeneous slab subjected to a bending moment will be zero and hence does not result in an output signal. Thus, single layer, plane parallel homogeneous active member slabs cannot be used independently as bending sensors. Homogeneous, plane parallel slab sensors require two separate regions. Two active piezoelectric regions, such as that depicted within a bimorph construction, may define the two regions, or the two regions may be defined by one active piezoelectric region bonded to one inactive region as depicted in a unimorph design. Since the second slab is crucial to the functionality of the device, all the complexities and problems described above in the fabrication and function of morph style actuators are also present in morph style sensors.
The architecture inherent in morph style actuators and sensors is difficult to implement in millimeter and sub millimeter devices. The difficulties come from using discrete components to obtain the regions of different piezoelectric activity required to make a unimorph or bimorph style device. The task of assembling individual components becomes burdensome as these discrete components become small and correspondingly delicate. As such, Micro-Electro-Mechanical Systems (MEMS) scale morphs are currently fabricated using sputtered and sol-gel films along with subsequent etching steps to define the outline. These deposition processes have associated problems. The elevated temperatures required during these processing steps lead to large residual stresses due to coefficient of thermal expansion, CTE, mismatches between the various layers required by morphs for actuation. In addition, film densification causes further shrinkage and leads to more residual stress. Hence, finished, room temperature devices exhibit large internal stresses, curling and other similar effects. These effects are accentuated in bimorph designs and consequently, bimorphs are not used in piezoelectric MEMS designs. In addition, for devices with significant internal stress, maximum allowable deflections may need to be reduced in order to remain within the material's allowable maximum stress. The CTE mismatch issues remain with the device making the actuator inherently sensitive to temperature changes in the environment. In addition, as devices get smaller, the thickness of bonding layers and inactive layers, caused by contamination or material composition changes during processing, get relatively bigger, increasingly impacting the efficiency of the device. For layered MEMS devices, layer to layer bonding failure represents a significant reliability problem.
Another characteristic of sub millimeter devices is that the stiffness or mechanical impedance of the devices diminishes rapidly due to the relationship between stiffness and member thickness. This reduced stiffness limits the application of small morph actuators to very low force application. In addition, it is difficult to find stable, non-active layers of a workable thickness sufficiently compliant relative to the stiffness of the active layer for efficient actuation.
Consequently for the fabrication of devices at the millimeter, sub millimeter and MEMS scale, there is a need for bender device architectures that reduce residual internal stress. Furthermore, there is a need to reduce the sensitivity of small bender devices to thermal changes. In addition, there is a need to increase the mechanical impedance of small morph style actuators. Still further, for small devices, there is a need for an architecture that does not require the assembly of discrete components.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to improve the characteristics of small size scale morph style bending actuators.
It is another object of the present invention to provide a simplified bending actuator and bending sensor.
A further object of the invention is to provide a bending actuator and sensor that has reduced internal stress.
It is yet another object of the present invention to provide a single slab, homogeneous material, bending actuator and bending sensor design for MEMS applications that does not suffer from deposition stress or metallic bonding failures.
A further object of the invention is to provide a bending actuator that has increased stiffness.
Another object of the present invention is to provide a method of making a single slab, homogeneous material bending actuator.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.